This is not the answer or help in the problem but hint how to make things other way.

You could consider using -n or -p switch to ease your script. I use it all the time and it really saves me much thinking about file names and handles and other stuff like error handling.

1. Reading. Instead of

#!perl
use strict;

open INPUT,...
while(<INPUT>)
{
...code inside loop...
}
[download]

and running like this:
C:\>myscript.pl
use this

#!perl -n
use strict;
...code inside loop...
[download]

and run like this
C:\>myscript.pl input_file.txt

2. Writing. Instead of opening CSV file and printing to it, print to STDOUT. And redirect to file like this:
C:\>myscript.pl > OUTPUT.csv
Combine those two together(-p switch will print $_ for you):

#!perl -n
use strict;
...code to do with every line of input stored in $_...
print $_; # or just print
[download]

or

#!perl -p
use strict;
...code to do with every line of input stored in $_...
[download]

Regardless which switch you choose, use the script like this:
C:\>myscript.pl input_file.txt > OUTPUT.csv

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